1. Field of the Invention
The present invention generally relates to vehicle occupant restraint systems that employ air bags. More specifically, the present invention relates to a propellant for producing nitrogen gas to inflate an air bag, a method for producing such a propellant, and an air bag vehicle occupant restraint system that utilizes the propellant.
2. Description of the Related Art
Generally, an air bag restraint system includes a reaction canister for containing a propellant that, when ignited, produces a gas that is used to inflate an air bag. To ignite the propellant, an igniter that is responsive to a signal provided by a collision sensor is disposed adjacent to the propellant. The air bag restraint system also includes an air bag that is located across an open end of the reaction canister. Located in the canister and between the air bag and the propellant is a filter for preventing the hot residues produced by combustion of the propellant from entering the air bag and possibly coming into contact with the occupant of the vehicle when the air bag is inflated. Briefly, operation of an air bag restraint system is initiated when the sensor detects an imminent collision or a collision and causes the igniter to ignite the propellant. Ignition of the propellant, in turn, produces gas to inflate the air bag and thereby prevent injuries to the occupant of the vehicle by restraining or inhibiting their movement. Shortly after inflation, the air bag deflates to permit the occupant to move and exit the vehicle if necessary.
Presently, the propellant most commonly used in air bag restraint systems includes sodium azide (NAN.sub.3), which produces nitrogen gas for inflating the air bag when combusted. Propellants that incorporate sodium azide, upon combustion, produce a gaseous atmosphere of substantially 100% nitrogen gas for inflating the air bag. This is advantageous because nitrogen gas is substantially inert and can be inhaled by humans for short periods of time without harmful effects. The use of sodium azide does, however, present several drawbacks. Namely, sodium azide is a Class B poison and easily hydrolyzed into hydrazoic acid, which is also toxic and explosive. Moreover, sodium azide reacts with heavy metals, like copper and lead, to produce a very sensitive explosive that can be easily ignited. Due to these factors, substantial precautions are necessary in the transportation of sodium azide, its incorporation into air bag systems, and in its eventual disposal when the air bag system in which it is incorporated is discarded that add to the overall expense of such systems. For example, special precautions must be taken during transportation of the sodium azide to prevent leakage of this toxic and reactive material into the environment. Additionally, the air bag restraint systems that incorporate a sodium azide-based propellant require containers for the propellant that are designed to prevent the sodium azide from contacting heavy metals, becoming hydrolyzed, and from leaking into the occupant environment of the vehicle. Moreover, during disposal of air bag restraint systems that employ sodium azide as a propellant, further precautions must be taken to prevent the sodium azide from coming into contact with heavy metals or becoming hydrolyzed. These drawbacks are further compounded by the increasing demand for passenger side air bags in automobiles that deploy air bags with volumes three to four times that of the driver's side air bags and therefore, require proportionately greater amounts of sodium azide propellant.
A further drawback associated with the use of sodium azide-based propellants in air bag restraint systems is that the process for producing such propellants is a relatively complicated process that involves blending the sodium azide with a refractory oxidizer, such as iron oxide or copper oxide, pressing the resulting mixture into pellets, and then establishing channels in the pellets for conducting gas and for controlling the burn rate of the propellant. The complexity of producing sodium azide-based propellants, in turn, increases the expense of the resulting air bag vehicle restraint systems that incorporate such propellants.
Yet a further disadvantage of sodium azide-based propellants is that it is difficult to design and manufacture the propellant with a desired burn rate. As previously mentioned, one proposed solution to controlling the burn rate of sodium azide-based propellants has been to establish channels in the pellets. The establishment of channels in the pellets adds to the complexity as well as the expense of manufacturing the propellant. Another proposed solution to controlling the burn rate of sodium azide-based propellants has been to incorporate graphite fibers into the propellant. This also requires additional manufacturing steps and increases the expense of the resulting propellant. Moreover, the degree to which the burn rate can be regulated by these solutions has, in many cases, proven to be subject to unacceptable variation.
Another disadvantage of sodium azide-based propellants is that combustion of the propellant produces a molten sodium oxide (Na.sub.2 O) residue that is in the form of relatively small globules with low viscosity. The relatively small size of the globules and their low viscosity necessitate the use of "slagging" agents to increase the viscosity of the residue and a relatively expensive filter to be interposed between the propellant and the air bag to prevent the residue from entering the air bag and possibly burning the occupant of a vehicle in which the restraint system is employed.
Alternatives to sodium azide-based propellants have been developed that are derived from hydroxamine and hydroxylamine. Further, propellants using polymeric binders, hydrocarbons, carbohydrates, and dialkali salts of bitetrazole and azobitetrazole have also been developed. Many of these alternatives to sodium azide-based propellants have many of the same disadvantages as the sodium azide propellants or other disadvantages that have not made it worthwhile to convert from the sodium azide-based propellants.
Based on the foregoing, there is a need for a propellant for an air bag vehicle restraint system that reduces the use of sodium azide to, in turn, reduce the problems associated with the transportation of sodium azide, the incorporation of sodium azide into air bag restraint systems, and the subsequent disposal of air bag restraint systems that employ sodium azide. There is the further need for a propellant for use in air bag restraint systems that can be easily and inexpensively manufactured. Moreover, a propellant for air bag systems is needed in which the burn rate of the propellant can be readily controlled or regulated. Moreover, a propellant for air bag restraint systems is needed that, upon combustion, produces a residue that reduces the need for a complicated or expensive filtering mechanism to prevent molten material or other residue produced by combustion of the propellant from entering the air bag and possibly injuring an occupant of the vehicle in which the air bag restraint system is installed.